Cationic direct dyes have been known for a long time in hair tints. In oxidative coloring systems, which simultaneously lighten the natural hair melanin during the coloring operation, nitro and azo dyes in particular are used since most other types of dye do not withstand the oxidizing process.
From JP-OS 07-126543 it is known to use certain azacyanines for the surface treatment of optical recording media. It is likewise known from European patent 0285000 to use certain cationic azacyanine dyes for detecting enzymes cleaving peptide bonds.